To add to radiantmatrix's list:

(most of these are less effective at stopping persistent abuser, but shouldn't stop any valid postings)

• robots.txt file to stop the innocent search engines from posting
• one time keys in hidden inputs to track multiple postings to the form without a refresh
• timestamps in hidden input to track the length of delay between form request and submission. (for some forms, this isn't appropriate, but you can refill the form and ask them to resubmit and/or do something to confirm they aren't a bot)
• input validation to ensure that the form hasn't been bypassed (eg, make sure select values are options that were on the form)
• user-agent filtering as there have in the past been signatures of known misbehaving bots, and you might be able to identify a single abusive system/signature
• rate limiting on all submissions to your system, rather than just a random per-submission delay. (so the more submissions to the site, the longer the delays introduced ... normally to slow down ballot stuffing so that admins can deal with it)

Oh and for the original poster -- and there are plenty of capchas that don't discriminate against visually-impaired, but may cause problems for some other subset of users. Some simple ones are math problems (arithmetic, not calculus) or 'spot the member that's different' where alt text can work (eg, 8 bird species and a dog breed). I've even seen 'write 2 in the box'. Of course, CAPTCHAs don't work. See If CAPTCHA isn't the answer. What is? for more details.

Oh -- and a timestamp hashed against the IP address makes a fairly effective combined one time key and timestamp.

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